1. Field of the Invention
The present invention relates to a pneumatic tire, more particularly to a heavy duty radial tire improved in bead durability without increasing the tire weight.
2. Description of Prior Art
In heavy duty radial tires for trucks, buses and the like, hitherto, in order to increase the bead durability, as shown in FIG. 10, a bead apex rubber (b), which is disposed between a main portion (a1) and turnup portion (a2) of a carcass, was increased in the volume to increase the rigidity of the bead portion, and thereby the deformation of the tire under load is decreased.
Further, in order to even the contact pressure between the bead portion and a flange of a wheel rim, the bead portion is provided on the axially outer surface with a concave part accommodated to the curved inner surface of the rim flange. Such a conventional bead structure is shown, for example, in the laid-open Japanese patent application No. JP-A-9-263113.
In recent years, a technique in which the volume and height of the bead apex rubber are considerably decreased has been proposed. In this technique (hereinafter slim bead structure), as the bead apex rubber thickness is decreased, the thickness of a chafer rubber (d) disposed axially outside the bead apex rubber (b) is relatively increased. This is preferable per se because this is particularly effective to increase the bead rigidity against axially outward bending.
However, if a tire having such a slim bead structure is provided with the above-mentioned concave part (e) as shown in FIG. 9, the bead durability is liable to decrease because, during vulcanizing the tire in a mold, the chafer rubber (d) is pressed by a protrusion of the mold provided for forming the concave part, and the axially outside part of the chafer rubber (d) flows radially outwards. As a result, the boundary (k) between the chafer rubber (d) and the sidewall rubber (f) undulates, and the bead durability is decreased and the rigidity distribution is disturbed.
Further, it was found that even when the concave part (e) is not formed, the same problemxe2x80x94undulations of the boundaryxe2x80x94arises, if a bead core (c) is moved axially outward during vulcanizing the tire, and the under part of a chafer rubber (d) is pushed and thus flows radially outwards as shown in FIG. 8. Also it was found that such a movement is closely related with the inclination angle of the axially outer surface of the bead portion near the bead heel, more particularly, the inclination angle during vulcanizing the, tire.
Therefore, an object of the present invention is to provide a heavy duty radial tire based on a slim bead structure, in which undulations of the boundary between a chafer rubber and a sidewall rubber are effectively prevented to improve the bead durability, without hindering a tire weight reduction.
In a slim bead structure, on the other hand, as the volume of the bead apex is decreased, above the bead core, a carcass ply turnup portion is curved steeper than ever. As a result, during building a raw tire, an air gap is liable to be formed between the carcass ply turnup portion and a bead apex rubber by spring-back of the carcass cords. This also greatly decreases the bead durability.
Therefore, another object of the present invention is to provide a heavy duty radial tire based on a slim bead structure, in which the occurrence of air gap effectively is prevented to improve the bead durability.
According to one aspect of the invention, a heavy duty radial tire comprises
a tread portion,
a pair of sidewall portions,
a pair of bead portions with a bead core therein,
a carcass comprising a ply of cords extending between bead portions and turned up around a bead core in each bead portion from the inside to outside of the tire so as to form a pair of turnup portions and a main portion therebetween,
a bead apex rubber disposed between each of the turnup portions and the main portion,
each of the turnup portions extending radially outwardly beyond a radially outer end of the bead apex to adjoin the main portion,
a chafer rubber disposed along at least an axially outer surface of each of the bead portions so as to define at least a flange-contacting part for contacting with a flange of a wheel rim,
the height of the radially outer end of the bead apex being in the range of from 7 to 35% of the height of the carcass at the tire equator, and being less than the height of the radially outer end of the chafer rubber,
the flange-contacting part being devoid of a concave profile.
In order to achieve the second object, the above-mentioned cords of the carcass ply are steel cords whose sectional area is in the range of from 0.10 to 0.25 sq.mm, and a carcass reinforcing layer is disposed along the carcass so as to extend between at least the maximum tire width points and the edges of a belt disposed radially outside the carcass in the tread portion.